Shameless self-promotion:
All Hail FBInk!.
As for an actual answer: eips is very particular as to how it wants a PNG to be encoded in order not to mangle it (IIRC, it expects what is roughly colloquially called PNG8. See the ScreenSavers' hack cover mode for all the nitty gritty details). That's probably what's happening here.
(FBInk is much more lenient on that front).